This invention relates to a noncontact-type displacement sensor for detecting displacement of a moving body undergoing rectilinear movement.
When a permanent magnet moving together with a moving body displaces relative to a magnetism-detecting device and the magnetic flux density detected by the magnetism-detecting device changes, it is possible to detect the displacement of the permanent magnet, and hence of the moving body, from a detection signal of the magnetism-detecting device. In a noncontact-type position sensor disclosed in U.S. Pat. No. 6,211,668 (which corresponds to JP-A-2000-180114), each of a pair of permanent magnets facing each other across an air gap is thinner at its center than at its ends in a movement direction. The magnets facing each other are magnetized in the same thickness direction and in opposite ways on either side of their centers, and consequently the flux density in the air gap changes in one direction from one end of the magnets to the other. The position of a magnetism-detecting device located in the air gap changes between one end of the magnets and the other, and the position sensor detects displacement of a moving body on the basis of variation in the flux density detected by the magnetism-detecting device.
However, in a position sensor of the construction disclosed in U.S. Pat. No. 6,211,668, when the angular position of the permanent magnets changes about the locus of movement of the magnetism-detecting device, which moves relative to the permanent magnets between the ends of the permanent magnets, the direction of the flux passing through the magnetism-detecting device changes. Consequently, the flux density detected by the magnetism-detecting device changes, notwithstanding that the magnetism-detecting device is not displacing relative to the permanent magnets along its locus of rectilinear movement.
Therefore, it is not possible to detect displacement in the rectilinear movement direction of a moving body that rotates while undergoing rectilinear movement. Even if the moving body is not intended to rotate, if a rotation-preventing mechanism for preventing rotation of the moving body is not used, the moving body may rotate while it moves rectilinearly. Since, to precisely detect displacement in the rectilinear movement direction of a moving body undergoing rectilinear movement, a rotation-preventing mechanism for preventing rotation of the moving body is necessary, the number of parts increases. Also, because the rotation-preventing mechanism slides against the moving body as the moving body moves, there is a risk that the moving body or the rotation-preventing mechanism will wear.